During downshifting or sudden throttling on a slippery road surface, the driven wheels of a vehicle may slip due to the engine braking effect. In order to continually ensure sufficient driving stability, the engine drag torque regulation (EDTR) generates additional engine torque as soon as a driven wheel slips and falls below a specified slip threshold. During the increase of the engine torque it must be assured that the clutch is engaged. The torque addition would otherwise only result in an increased engine speed with accompanying extreme engine noise.
Declutching during engine drag torque regulation occurs mostly at the beginning of the regulation when the driver is startled because the vehicle starts to slip after downshifting, but also at sudden throttling on a slippery road surface—in particular when traveling through curves—and the stabilizing reaction of the EDTR is not yet noticeable due to the delay of the system.
The extreme engine noise is sensed by the driver as at least unpleasant, but it may also startle the driver, thereby creating dangerous situations.
The use of clutch switches for detecting the clutch state is known. However, these switches are relatively expensive and require individual wiring.
Determining the clutch state by comparing the current overall gear ratio with the overall gear ratio of the first gear is also known. If quotient Nmot/vman, proportional to the overall gear ratio (Nmot: engine speed; vman: average speed of the driven wheels), is greater than the overall gear ratio of the first gear, then a declutched state is identified.
This method of detecting the clutch state is clearly shown in FIG. 1. Engine speed Nmot is plotted in the figure over the average speed of the driven wheels vmna for different gears. The characteristic lines for the individual gears are marked with reference numbers 5-8 (first gear 5 through fourth gear 8).
The overall gear ratio Iges is 10 in the first gear for example and 3.7 in the third gear for example. A declutched state is identified in this case when the determined quotient Nmot/vman lies above the quotient of the first gear (thus in the entire area 9); in order to prevent interfering effects, a certain distance (dashed line 10) to the gear ratio of first gear 5 is adhered to.
In order for this declutch detection to respond, the engine speed, driven by the EDTR increase momentum, increases considerably. The lower the gear step (second, third, or fourth gear), thus the lower the overall gear ratio where declutching takes place, the more the speed increases so that the declutch detection responds. This is associated with a corresponding surge of the engine noise.